GEOL212: Planetary Geology
Fall 2017

Life in the Solar System - Let's be reasonable - II

Since Cassini/Huygens' arrival at Titan (2004) we have seen that it fills the need for at least some of the requirements of life:

Liquid ethane/methane: A liquid medium in which life processes can unfold, pooled in large lakes and present in the soil.

Abundant organic molecules: The building blocks of life that can be recruited by living systems. Indeed, we suspect that interesting organic chemistry goes on in these lakes simply because they are so viscous that waves don't form on their surfaces, even though winds blow over them. Pure ethane and methane should not be so "gummy." (Kirichek et al., 2012.)

These observations alone were sufficient to fuel speculation by authors such as McKay and Smith, 2005 about methanogens, possible life forms living in a methane/ethane medium.

At 97 K, Titan simply has much less thermal energy than Earth or Mars. Could Titan's low-energy environment possibly support an energy-processing biosphere?

Pre-biotic mix: Overall, Titan looks like a very cold version of the early Earth, prior to the rise of life. Oceans filled with dissolved organic molecules waiting to be assembled into living things. Unlike Earth's oceans, Titan's may have been kept in a deep-freeze for the history of the Solar System, impeding the rise of life.

Hydrogen/Acetylene surprise: Does Titan have a chemical pathway that could conceivably power living systems? Maybe. Like Viking, Huygens has presented some tantalizing if inconclusive data. Huygens was equipped with a GCMS instrument. As is hung on its parachute, its GCMS sniffed the air, compiling profiles of the atmospheric densities of gasses at different elevations ( Niemann et al., 2005). This profile was surprising in two major ways:

Hydrogen (H2): No surprise that hydrogen was depleted in Titan's upper atmosphere - it was leaking into space. The surprise was that it was also depleted near the surface. Where is it going?

Acetylene (C2H2): Atmospheric models predicted that this hydrocarbon would be generated in the atmosphere and accumulate on the surface. Instead, its concentration also diminished near the surface. Strange.

This has fueled speculation by Strobel, 2010 that these chemicals are being consumed by methanogenic life forms. Indeed, at this moment, the methanogen hypothesis looks like the strongest one on the table.

Earth Biosphere Analogy:

On Earth, most life is powered by the oxidation of the sugar glucose by oxygen in this reaction:

C6H12O6 + 6O2 --> 6H2O + 6CO2

This is misleading because at Earth temperatures this reaction doesn't take place without a complex set of catalysts. In contrast, if we put hydrogen and acetylene together, we get a vigorous reaction:

3H2 + C2H2 --> 2CH4

Like the glucose oxidation, the oxidation of acetylene by hydrogen releases considerable energy. Arguably, the depletion of these chemicals near Titan's surface means that they are being consumed by some surface chemical process. But the cool thing: At Titan temperatures, this reaction can only proceed in the presence of complex catalysts.

This doesn't "prove" or "confirm" anything about the presence of life in Titan's liquids, but it is highly suggestive. Either some sort of catalyzed metabolic or protometabolic process is using up the H2 and C2H2 or some completely unknown inorganic reaction is doing the same. Either way, Titan just got lots more interesting.

On a more speculative note, a water ocean is thought to exist deep beneath Titan's icy crust. Could it, too harbor life? There is no evidence at all to support this, but it is interesting to imagine that Titan might support two totally separate biospheres.

Now, suppose we could identify an environment identical to the hydrothermal vent environments of the ancient Earth in which terrestrial life arose? The prospects for life in such an environment ought to be good. That place is Europa.

Europa is a rocky world encased in a thin layer of water and ice. The thickness of the ice is currently unknown, but the fact that the icy crust rotates at a different rate from the interior indicates that the two solid regions are separated by a liquid mantle.

The presence of a global magnetic field suggests that the water is saline, with currents.

Europa experiences tidal flexing that ought to cause internal heat. This could be manifested as hydrothermal vent environments at the bottom of the liquid mantle.

Thus, we have an environment that provides the same:

Liquid medium

Energy source

that the first terrestrial microbes enjoyed. For this reason:

Europa is generally regarded as the most likely place in the Solar System to harbor life.

Oxygen: Adding color to this assessment is the assertion by Greenberg, 2010 that exposure of surface ice to cosmic rays would convert it to oxidizers. If the ice is thin enough to allow these to be transported into the liquid mantle/ocean, it may result in its widespread oxygenation. In such an event, life would be able to proliferate throughout the mantle/ocean, and it could support large, complex, energy intensive life forms. (An imaginary example appears at right). Indeed, Europa seems like the only alien world in the Solar System with any chance of supporting more complex life than simple microbes.

Europan lakes?: Schmidt et al., 2011 propose that Europa's chaos regions result from rising lenses of liquid water interacting with the surface. If true, it strengthens the idea that the transfer of material between the deep ocean and surface is possible.

Before you get too excited: Currently, no speculation on Europan life is supported by any data. We have only conjecture based on the perceived conduciveness of an environment that may or may not exist. Potential pitfalls:

The liquid mantle/ocean may actually be a layer of soft convecting ice.

Even if the water is present, the icy crust may be so thick that exchange of oxidizers from the surface would be impossible, limiting life to the regions around hydrothermal vents.

We have no idea what is dissolved in this water, although the magnetic field suggests that there are ions in solution. Some speculation requires significant quantities of sulfuric acid. Ouch.

But the real tribute to Europa's promise is that it has become the focus of the next generation of high-profile "flagship" mission concepts by NASA and ESA.
Major priorities of these Europa missions include:

Characterizing its tidal flexing. (I.e. determining how much energy is there.)

Determining the thickness of the ice. (Could oxidizers make it into the mantle/ocean?)

Identifying substances on the surface, which should include material extruded from the mantle/ocean during impacts and the formation of lineae.

A simpler option? But wait! Many other icy worlds are thought to harbor sub-surface water. The problem is getting to it. On these worlds (E.g. Ganymede), the water, if it exists, is at very great depth.

But on Enceladus, we know about the water because we see it erupting. Clearly, it has enough energy at depth to power its cryovolcanoes. Could life have appeared in this deep environment? If it has, we might be able to sample it from the surface, because Enceladus is one place where it might actually be snowing microbes. In fact, mission concepts to: